Here according to the equation we need to see how the constraint works here actually..
Given ,
f(x,y,z) = f(x',y,z')
So f(0,0,0) = f(1,0,1)
f(0,0,1) = f(1,0,0)
f(0,1,0) = f(1,1,1)
f(0,1,1) = f(1,1,0)
So we can see that each minterm(possible combination of x,y,z) is covered exactly once..
Now say f(0,0,0) can be 0 or 1..But
f(0,0,0) = f(1,0,1)
So having selected f(0,0,0) we have one choice for f(1,0,1)..
Likewise for other 3 pairs also..
Hence number of functions possible = 2 * 2 * 2 * 2 [ 2 choices for each pair ]
= 16 functions
Hence B) should be the correct answer..